Our main objectives are to continue our studies on poly ADP-ribose polymerase along two broad lines: first, dealing with relationships between poly ADP-ribosylation of chromosomal proteins and functions in chromatin; and second, continuing our studies on the overall importance of this nuclear protein modification on chromatin structure and architecture. Our strategy during the last year has been to approach poly ADP-ribosylation at the oligonucleosomal level of chromatin and to systematically catalog: (1) the domains of chromatin possessing high activity for the modification; (2) the major nuclear protein acceptors for the modification; and (3) the influence of polymer chain length on the structure of these acceptors and subsequently on the structure of simple oligonucleosomes. These studies have yielded new information concerning chromatin structure, per se. (1) HeLa nuclear polymerase has been purified to homogeneity and characterized. (2)\Oligonucleosome-reconstitution techniques have been established for purified polymerase. (3)\Antibody to polymerase has been produced and used to titrate the association of the enzyme with different types of nucleosomes. (4) A NAD-dependent aggregation of oligonucleosomes and cross-linking nuclear proteins has been characterized. (5) Poly ADP-ribose cross-linked dimer of histone H1 has been studied in vivo and in purified nucleosomes. By reconstitution experiments, poly ADP-ribose cross-linking of H1 has been shown to be important for the condensation of chromatin subunits. (6) We have prepared and characterized an immunoaffinity column using antibody specifically directed against poly ADP ribose. This antibody column was used to isolate the small domain of chromatin undergoing poly ADP-ribosylation. We have also used this column to show that histone H1 molecules become mutually poly ADP-ribosylated and phosphorylated by histone kinase.